We are interested in the isolation and characterization of conditionally lethal mutations in mammalian cells in culture. Two classes are being studied: 1) temperature-sensitive mutants defective in functions essential for cell proliferation, and 2) mutants defective in respiratory activity, e.g. in the Krebs cycle, which as a result become auxotrophs for carbon dioxide and perhaps other small molecular weight compounds under defined conditions. We hope that mutants in the first class will help us understand in more detail the regulation of the "biochemical differentiation" during the cell cycle, in particular the series of events which constitute the early part of the commitment of a cell towards DNA replication and cell division. Such mutants might help us test new models of the cell cycle for mammalian cells. A mutant of the second class has been isolated in our laboratory and we propose to find others. However, initially the emphasis will be on a complete biochemical and genetical analysis (by somatic cell hybridizations) of the existing mutant, which we have already shown to be almost totally defective in respiration and extremely active in glycolysis. It is very reminiscent of Warburgs ideas about cancer cells, but in our case the phenotype is much more extreme than that of any tumor cells studied, and we hope to demonstrate that we are dealing with a true mutation. Nevertheless, the mutant promises to be extremely interesting and helpful to our understanding of the regulation of glycolysis and respiration in normal and malignant cells.